1. Field of the Invention
The present invention relates generally to the production of bent sheets of glass, and more particularly to an improved method and apparatus for press bending glass parts.
2. Background of the Invention
Curved or bent sheets of glass are commonly used as glazing closures in automotive vehicles and the like. It is imperative that the glass sheets be bent to precisely defined curvatures determined by the configuration and size of the vehicle openings, as well as the overall styling of the vehicle. At the same time, the bent sheets must meet stringent optical requirements, i.e., the viewing area of the closure or window must be free from optical defects that would tend to interfere with clear viewing therethrough. Thus, it is necessary that the bending apparatus not only shape the glass sheets to precise curvatures, but also that it do so without adversely affecting their optical quality.
Over the years as automotive styling has evolved, the shapes to which the sheets must be bent have become increasingly complex, and thus the sheets have become more difficult to efficiently produce on a mass-production basis. Such curved or bent sheets are now generally produced by a so-called "press bending" technique wherein flat glass sheets are heated to a temperature corresponding substantially to the softening point of the glass. The heated sheets are then pressed or shaped to the desired curvature between male and female mold members having complementary shaping surfaces. The bent sheets are cooled in a controlled manner to either anneal or temper the glass as dictated by their intended end use. Such press bending may suitably be carried out with the sheets oriented vertically, horizontally or obliquely. To achieve a high degree of accuracy in the configuration of the curved sheet while minimizing the application of pressure to the glass surfaces in their viewing area by the press members, the male or convex mold member is generally constructed with a solid or continuous shaping surface which contacts the entire surface of the sheet. Conversely, the cooperating female or concave mold member is constructed with an outline or ring-type shaping surface which engages only the surface of the peripheral marginal edge portions of the sheet.
In such press bending operations the female ring, prior to contacting the glass during the pressing operation, may be at a substantially lower temperature than the preheated glass. This is particularly true when the press bending is performed outside of the furnace. Consequently, when the heated glass contacts the relatively cooler bending ring, heat is transferred from the hot glass to the cooler bending ring, preferentially cooling the glass in these areas. As a result, the temperature of the glass in these localized areas is greatly reduced, causing tensile stresses to be created inboard of but near the bending ring. If the differences in temperature are great enough, the tensile stresses may result in chill cracks being produced in the glass sheet. In any event, the near-edge tensile stresses decrease the strength and durability of the glass sheet, leading to increased breakage after production.
In the past, efforts have been made to reduce the amount of tensile stresses formed in this way. For example, in accordance with U.S. Pat. Nos. 3,253,898 and 3,278,288, a gravity bending mold is provided with resistance type heating elements along the shaping rail to aid in the controlled cooling of the glass during annealing. Such gravity molds bend glass sheets utilizing heat and gravity, and the amount of bend that may be produced in the sheets is limited. Due to styling advances, there is an ever-increasing demand in the automotive industry for windshields having deeper bends and bends of smaller radius than can be satisfactorily achieved using gravity bend methods. Consequently, a great deal of developmental work has been directed to the press bending of windshields. Since windshields normally are fabricated from relatively thin sheets of glass (less than 3mm), they tend to cool rapidly and are especially prone to chill cracking and reduction of edge strength due to the formation of tensile stresses caused by contact with the female bending ring.
U.S. Pat. No. 3,837,833 discloses a cloth-covered shaping rail in a press bending operation that is provided with heated metal cover protectors located in areas that are more susceptible to wear. The purpose of the metal cover protectors is to prolong the useful life of the cover cloth. The stainless steel metal protector is electrically heated to eliminate any tendency for thermal shock to occur in the glass sheet when contacting the exposed metal protectors.